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The class of transition metal phosphates (TMPs) shows a wide range of chemical compositions, variations of valence states and respective crystal structures. Among TMPs, $VO(P_2O_7)$ and $LiFePO_4$ are of special interest as the only commercially used heterogeneous catalyst for the selective oxidation of butane to maleic anhydride [1] and cathode material in rechargeable batteries [2]. Due to their structural features, TMP are considered as proton exchange-membranes in fuels cells, working in the intermediate-temperature range [2, 3]. We report on the successful ab initio structure determination of two novel titanium pyrophosphates, $NH_4Ti(III)P_2O_7$ and $Ti(IV)P_2O_7$, from X-ray powder diffraction data. Both compounds were synthesized via a new molten salt synthesis route. The low symmetry space groups $P2_1/c$ ($NH_4TiP_2O_7$) and $P\overline{1}$ ($TiP_2O_7$) complicate the structure determination, making the combination of spectroscopic, diffraction and computation techniques mandatory. In $NH_4TiP_2O_7$, titanium ions ($Ti^{3+}$) occupy the $TiO_6$ polyhedron, coordinated by five pyrophosphate groups, one as a bi-dentate ligand. This secondary coordination causes the formation of one-dimensional six-membered ring channels with a diameter $d_{max}$ of 514(2) pm, stabilized by ammonium ions. Annealing $NH_4TiP_2O_7$ in inert atmospheres results in the formation of the new $TiP_2O_7$, showing a similar framework consisting of $[P_2O_7]^{4-}$ units and $TiO_6$ octahedra as well as an empty one-dimensional channel ($d_{max}$ = 628(1) pm). The in situ X-ray diffraction study of the transformation of $NH_4TiP_2O_7$ to $TiP_2O_7$ reveals a two-step mechanism, the decomposition of ammonium ions coupled with the oxidation of $Ti^{3+}$ to $Ti^{4+}$ and a subsequent structural relaxation.
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